From: "Michael S. Tsirkin" <mst@redhat.com>
To: Rafael Aquini <aquini@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
Rusty Russell <rusty@rustcorp.com.au>,
Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>,
Andi Kleen <andi@firstfloor.org>,
Andrew Morton <akpm@linux-foundation.org>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility
Date: Tue, 21 Aug 2012 22:13:30 +0300 [thread overview]
Message-ID: <20120821191330.GA8324@redhat.com> (raw)
In-Reply-To: <20120821172819.GA12294@t510.redhat.com>
On Tue, Aug 21, 2012 at 02:28:20PM -0300, Rafael Aquini wrote:
> On Tue, Aug 21, 2012 at 09:24:32AM -0700, Paul E. McKenney wrote:
> > On Tue, Aug 21, 2012 at 05:20:11PM +0200, Peter Zijlstra wrote:
> > > On Tue, 2012-08-21 at 09:47 -0300, Rafael Aquini wrote:
> > > > + mapping = rcu_access_pointer(page->mapping);
> > > > + if (mapping)
> > > > + mapping = mapping->assoc_mapping;
> > >
> > > The comment near rcu_access_pointer() explicitly says:
> > >
> > > * Return the value of the specified RCU-protected pointer, but omit the
> > > * smp_read_barrier_depends() and keep the ACCESS_ONCE(). This is useful
> > > * when the value of this pointer is accessed, but the pointer is not
> > > * dereferenced,
> > >
> > > Yet you dereference the pointer... smells like fail to me.
> >
> > Indeed!
> >
> > This will break DEC Alpha. In addition, if ->mapping can transition
> > from non-NULL to NULL, and if you used rcu_access_pointer() rather
> > than rcu_dereference() to avoid lockdep-RCU from yelling at you about
> > not either being in an RCU read-side critical section or holding an
> > update-side lock, you can see failures as follows:
> >
> > 1. CPU 0 runs the above code, picks up mapping, and finds it non-NULL.
> >
> > 2. CPU 0 is preempted or otherwise delayed. (Keep in mind that
> > even disabling interrupts in a guest OS does not prevent the
> > host hypervisor from preempting!)
> >
> > 3. Some other CPU NULLs page->mapping. Because CPU 0 isn't doing
> > anything to prevent it, this other CPU frees the memory.
> >
> > 4. CPU 0 resumes, and then accesses what is now the freelist.
> > Arbitrarily bad things start happening.
> >
> > If you are in a read-side critical section, use rcu_dereference() instead
> > of rcu_access_pointer(). If you are holding an update-side lock, use
> > rcu_dereference_protected() and say what lock you are holding. If you
> > are doing something else, please say what it is.
> >
> > Thanx, Paul
> >
> Paul & Peter,
>
> Thanks for looking into this stuff and providing me such valuable feedback, and
> RCU usage crashcourse.
>
> I believe rcu_dereference_protected() is what I want/need here, since this code
> is always called for pages which we hold locked (PG_locked bit).
It would only help if we locked the page while updating the mapping,
as far as I can see we don't.
> So, it brings me
> to ask you if the following usage looks sane enough to fix the well pointed issue,
> or if it's another misuse of RCU API:
>
> + mapping = rcu_dereference_protecetd(page->mapping, PageLocked(page));
> + if (mapping)
> + mapping = mapping->assoc_mapping;
>
next prev parent reply other threads:[~2012-08-21 19:12 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-21 12:47 [PATCH v8 0/5] make balloon pages movable by compaction Rafael Aquini
2012-08-21 12:47 ` [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility Rafael Aquini
2012-08-21 13:52 ` Michael S. Tsirkin
2012-08-21 14:25 ` Michael S. Tsirkin
2012-08-21 15:16 ` Peter Zijlstra
2012-08-21 15:41 ` Michael S. Tsirkin
2012-08-21 17:42 ` Rafael Aquini
2012-08-21 19:28 ` Michael S. Tsirkin
2012-08-21 17:55 ` Rafael Aquini
2012-08-21 19:16 ` Michael S. Tsirkin
2012-08-21 19:34 ` Rafael Aquini
2012-08-22 0:06 ` Michael S. Tsirkin
2012-08-21 15:20 ` Peter Zijlstra
2012-08-21 16:24 ` Paul E. McKenney
2012-08-21 17:28 ` Rafael Aquini
2012-08-21 19:13 ` Michael S. Tsirkin [this message]
2012-08-21 19:23 ` Rafael Aquini
2012-08-21 19:30 ` Michael S. Tsirkin
2012-08-21 20:45 ` Rafael Aquini
2012-08-22 0:07 ` Michael S. Tsirkin
2012-08-22 1:19 ` Rafael Aquini
2012-08-22 9:33 ` Michael S. Tsirkin
2012-08-23 2:19 ` Rafael Aquini
2012-08-23 10:01 ` Michael S. Tsirkin
2012-08-23 12:13 ` Rafael Aquini
2012-08-23 12:34 ` Michael S. Tsirkin
2012-08-23 13:06 ` Rafael Aquini
2012-08-23 13:53 ` Michael S. Tsirkin
2012-08-23 15:21 ` Rafael Aquini
2012-08-23 15:54 ` Michael S. Tsirkin
2012-08-23 16:03 ` Rik van Riel
2012-08-23 16:06 ` Rafael Aquini
2012-08-23 16:10 ` Michael S. Tsirkin
2012-08-23 16:25 ` Michael S. Tsirkin
2012-08-23 17:28 ` Rafael Aquini
2012-08-23 17:59 ` Rik van Riel
2012-08-23 23:36 ` Michael S. Tsirkin
2012-08-24 0:26 ` Rafael Aquini
2012-08-24 0:33 ` Rafael Aquini
2012-08-24 0:38 ` Rafael Aquini
2012-08-24 0:49 ` Rafael Aquini
2012-08-24 3:12 ` Rik van Riel
2012-08-24 8:03 ` Michael S. Tsirkin
2012-08-21 12:47 ` [PATCH v8 2/5] mm: introduce compaction and migration for ballooned pages Rafael Aquini
2012-08-21 12:47 ` [PATCH v8 3/5] virtio_balloon: introduce migration primitives to balloon pages Rafael Aquini
2012-08-21 14:40 ` Michael S. Tsirkin
2012-08-21 15:34 ` Peter Zijlstra
2012-08-21 15:37 ` Peter Zijlstra
2012-08-21 14:57 ` Michael S. Tsirkin
2012-08-21 12:47 ` [PATCH v8 4/5] mm: introduce putback_movable_pages() Rafael Aquini
2012-08-21 14:42 ` Michael S. Tsirkin
2012-08-21 12:47 ` [PATCH v8 5/5] mm: add vm event counters for balloon pages compaction Rafael Aquini
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